I would urge you to go the extra mile and op for
Canon. It's only the Rebel XT and XTi that are a tad on the small side. Even those are fine with the battery grip fitted.
If it really has to be the Nikon or the Pentax ..... then unfortunately I would have to suggest the Nikon. (Beware the D40 is even smaller than the Canon XT and XTi)
Happy shooting
Dave
Sounds like a Canon fanboy, professional or not.
Just do what they do in competitions and cut out the most extreme sounding post on both ends (ie: the most pro and con Nikon posts, and most pro and con Canon posts)
I have a Pentax *ist DL and bought a D50 for my dad a few months ago.
When you push the ISO to 1600 on the DL you get some more noise than you do on the D50, but I find the manual controls a lot more intuitive on the Pentax. I've not used the K100D, but I'd imagine the noise issue might be lessened by the shake reduction -- you'll be able to shoot at a lower ISO in low-light, plus there seems to be noise reduction (according to the features listed on the B&H web site)
Well Pentax cameras don't seem to do much in-camera noise reduction. This is a good or bad thing depending on how you look at it. After all, with most noise reduction, you quite a lot of detail in the photo, so if you want smeared out, low res photos, you want extreme noise reduction. I'd rather have something in between, but the Pentax way of not doing much noise reduction isn't bad depending on what you want.
The D50 is about the same size as the smaller Canon bodies.
And the D40 is even smaller !!!
Dave
Actually, the D50 is bigger and more comfortable than the 300D, 350D, or 400D. The D40 is around the same size, but more comfortable to hold. The Pentax K100D and K10D are VERY comfortable to hold as well. As a prosumer/hobbyist camera, I'd consider a Pentax for sure. They have lots of lenses, and the 3rd party lenses (from Sigma, Tamron, Tokina) are also available. Plus, like others have said, it has shake reduction built-in.
I'd get a D50 rather than a D40 because of the large number of fantastic 3rd party lenses available for a lower cost than the Nikon (and maybe Pentax) alternatives, most of which won't work with a Nikon D40. Actually, the D50 would be highest on my list, then the K100D, then the D40. If you're not pro, and you're not going to go pro and aren't interested in $1500+ "extreme" lenses, and you can get over that low-end Canon style body, I'd get the Nikon D50, Pentax K100D, Canon 400D, then finally the Nikon D40 (although it's really not a bad camera in terms of feel and photographic quality). This is based purely on what you have told us.